Linear accelerator

ABSTRACT

Apparatus for treatment by radiotherapy, comprises a patient table, a directional radiation source directed generally toward the patient table, the table and source being movable relative to each other at least axially with respect to the table and rotationally around the axis, the source being directionally adjustable within at least a plane including the longitudinal axis of the table, and including means to correlate axial relative motion of the table and directional adjustment of the source. Thus, the source of such an apparatus can be rotated about the table to access the predetermined position along a series of cones centered on the predetermined position and of the variable angle. Thus, the apparatus effectively accesses the predetermined position via lines of latitude rather than longitude. It is preferred if the source is directionally adjustable across an included angle of at least 20°, preferably more than 30°. The source can be (for example) held within a ring member centered substantially on the patient table.

The present invention relates to a linear accelerator for therapeuticuse.

The overall geometry of a linear accelerator is ideally constructed soas to enable the output beam of the radiation source to be directed atany chosen site within the patient. In order to avoid excessive dosesbeing applied to healthy tissue, the incident direction is normallyvaried throughout the treatment period, the various directions eachconverging on a single point. This single point thereby receives a fulldose whilst the surrounding areas receive only a minimal dose.

For this reason, the radiation source is normally rotatable about thelongitudinal axis of the table. In our application WO 97/13552, wedescribed a system in which the table was rotatable about its verticalaxis. This provided a system in which access to a designated area withinthe patient could also be obtained via a plurality of directions lyingalong lines of longitude. This permitted a combination of rotations ofthe source and of the table, further reducing the dose applied tosurrounding areas. However, in this arrangement, the range of directionsis limited by the maximum relative rotation of the table. This rotationmust be limited in order to prevent the table obstructing the rotationalpath of the radiation source.

Another system available from the applicant involves a plurality ofsmall Cobalt sources arranged in the sphere and directed toward thecentre of that sphere. Each source can be concealed or revealed via asuitable shutter, allowing control over dosage aspect. However, it isimpractical to provide further collimation of individual beam sourcesdue to the sheer number of individual sources present. Also, the systemis in practice limited to use for treating the head as the Cobaltsources need to be near the patient in order to be effective. It is alsoimpractical to add an imaging structure.

The present invention therefore provides an apparatus for treatment byradiotherapy, comprising:

a patient table

a directional radiation source directed generally toward the patienttable,

the table and source being movable relative to each other at leastaxially with respect to the table and rotationally around that axis,

the source being directionally adjustable within at least a planeincluding the longitudinal axis of the table;

and including means to correlate axial relative motion of the table anddirectional adjustment of the source.

Thus, the source of such an apparatus can be rotated about the table toaccess the predetermined position along a series of cones centred on thepredetermined position and of the variable angle. Thus, the apparatuseffectively accesses the predetermined position via lines of latituderather than longitude. This largely eliminates the inaccessible zoneprovided by the system of WO 97/13552, and avoids a single convergencepoint directly above and below the predetermined point. it also providesa single radiation source which can therefore be collimated to provideaccurate beam shaping and intensity control.

Thus, whilst existing arrangements maintain an isocentre fixed relativeto the source just above the patient table, the present invention doesnot, instead compensating by movement of the patient table.

It is preferred if the source is directionally adjustable across anincluded angle of at least 20°, preferably more than 30°.

The source is suitably held within a ring member centred substantiallyon the patient table.

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying Figures, in which:

FIG. 1 is a plan view of a known linear accelerator;

FIG. 2 is an illustration of the available irradiation directions of theapparatus of FIG. 1 in the direction of arrow A of FIG. 1;

FIG. 3 is a side view of a linear accelerator according to the presentinvention; and

FIG. 4 is an illustration of the available irradiation directions of theapparatus of FIG. 3, in the direction of arrow B of FIG. 3.

Referring to FIG. 1, illustrating a known arrangement, a ring shapedmember 10 includes within its annulus a linear accelerator directedtowards the centre of the ring. The linear accelerator can be rotatedwithin the ring so as to take up any position and thereby direct a beamof radiation toward the centre of the ring from any direction. A patienttable 12 is placed so as to pass through the ring at an angle α,positioned slightly below the centre of the ring 10 such that a patientplaced on the table will intersect that centre.

During treatment, the linear accelerator is rotated continuously aboutthe ring 10, such that the directional approach of the radiation beampasses through all directions in the plane defined by the ring 10. Thisserves to concentrate the radiation dose at the centre of the ring andminimise it in the surrounding tissue.

The angle α can be varied, effecting rotation of the patient table abouta vertical axis passing through the centre of the ring 10. This is doneduring treatment, so that, relative to the patient, the plane of theradiation beam direction rotates. This further minimises the doseapplied to surrounding tissue without affecting the dose applied to thefocal point of the beam.

The physical arrangement of the apparatus does however impose alimitation on the maximum and minimum values of α. Put simply, rotationof the table beyond these limits will cause it to foul the ring 10.Furthermore, the plane of the radiation beam direction always passesthrough the line which passes vertically through the focal point,meaning that the tissue immediately above and below the tumour receivesa higher dose, although much reduced when compared with the tumour. Thisis illustrated in FIG. 2, in which a sphere 14 is irradiated,representing for example an idealised human head. Lines 16 show the pathtaken by the radiation beam, assuming that the angle α is increasedincrementally by discrete intervals. In a genuine treatment session, αwould of course be varied as smoothly as possible.

It will be seen that there is a gap 18 which is not irradiated. Thismeans that the “unwanted” irradiation must be concentrated into the area20. Furthermore, there is a point 22 above and below the focal point atwhich the lines 16 all intersect and at which precaution must be takento avoid applying a higher dose.

Whilst in practice these limitations do not hinder the medicaleffectiveness of the system, it is always preferable to reduce thedosage to healthy tissue. This can be achieved via the apparatusillustrated in FIG. 3.

In FIG. 3, a ring 110 contains a linear accelerator which can be bothrotated around the ring and tilted by an angle θ away from the vertical.A patient table 112 is positioned so as to intersect the ring, such thata patient on the table 112 will lie over the centre of the ring 110. Thetable 112 is provided with a translation means 114 which is able to movethe table longitudinally within the ring, i.e. from left to right inFIG. 3.

During a treatment, the linear accelerator is rotated within the ring110 in the same way as the apparatus of FIG. 1. However, rather thanrotate the table 112, it is translated within the ring in correlationwith a variation in the angle θ to compensate. Thus, at all times thefocal point of the radiation beam remains fixed relative to the patient.The directional approach of the radiation beam is therefore a conerather than a plane. This can be illustrated in FIG. 4, correspondinggenerally to FIG. 2, in which a sphere 116 shows lines 118 illustratingthe point of contact between the irradiation direction and the surfaceof the sphere 116. It can be seen that these described a series ofconcentric circles on the surface of the sphere 116, which are more akinto lines of latitude in that there is no point at which they meet. Thereis a gap 120 caused by the maximum value of θ, but this can be limitedby suitable design in view of the physical space available for theapparatus, and is not an inherent design limitation. This thereforelimits the area into which “unwanted” irradiation must be confined, andavoids any concentrated areas.

It will be understood that variations can be made to the above-describedembodiment, without departing from the scope of the present invention.

What is claimed is:
 1. Apparatus for treatment by radiotherapy,comprising: a patient table, a directional radiation source directedgenerally toward the patient table, the table and source being movablerelative to each other at least axially with respect to the table androtationally around that axis, the source being directionally adjustablewithin at least a plane including the longitudinal axis of the table;and including means to correlate axial relative motion of the table anddirectional adjustment of the source.
 2. Apparatus according to claim 1wherein the source can be rotated about the table to access apredetermined position along a series of cones centered on thepredetermined position and of variable angle.
 3. Apparatus according toclaim 1, wherein the source is directionally adjustable across anincluded angle of at least 20°.
 4. Apparatus according to claim 1,wherein the source is directionally adjustable across an included angleof at least 30°.
 5. Apparatus according to claim 1, wherein the sourceis held within a ring member centered substantially on the patienttable.